1. In the wild, red grouse live largely on heather, a high-fibre (25%), low-protein (7%) food. Digestibility trials were carried out under semi-natural conditions, with magnesium as a digestibility marker. Two trials were done, one in autumn and one in spring.
2. Digestibility of the dry matter varied from 21 to 30% and metabolizable energy from 1.1 to 1.6 kcal/g. These variations were inversely related to intake and could partly be accounted for by facultative variations in holocellulose and lignin digestion.
3. Digestion of soluble carbohydrates, protein (measured as α-amino-nitrogen) and holocellulose varied between trials according to the initial concentration in the food. The digestibility of soluble carbohydrate was high (78–83%) in autumn (16% in food) and low (61–66%) in spring (11% in food) and that of protein was relatively low (24–31%) in autumn (6% in food) and high (42–48%) in spring (7% in food). Digestibility of crude fat was 30–33% for four birds and 20% for one bird.
4. By comparison with poultry, voluntary intake of dry matter was very high relative to body-weight and intake of energy appeared to be adequate. None the less, all birds lost weight during the trials, presumably for reasons other than energy shortage.
5. Urate excretion increased in parallel with body-weight losses, but formed only 2% of the total N output at low weight losses, in which event the main nitrogenous compounds in the droppings were α-amino N (presumably largely from undigested protein), ammonium salts and ornithuric acid.
6. The ornithuric acid was presumably a detoxication product of prolignins and possibly tannins and other polyphenols. Its excretion by grouse corresponds to the excretion of hippuric acid by ruminants.